1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device such as a power transistor constructed by triple diffusion.
2. Description of the Related Art
FIG. 5 is a fragmentary plan view of a known semiconductor device, in particular a power transistor constructed by triple diffusion, FIG. 6 is a sectional side elevational view of the semiconductor device shown in FIG. 5 taken along line 6--6. Referring to these Figures, a semiconductor substrate S has one conductivity type, e.g., N.sup.- type, and a base region 2 of the other conductivity type, e.g., P type, is located at on one principal surface, e.g., obverse surface, of the semiconductor substrate S. An emitter region 3 of N type is formed in the base region 2 by diffusion. An emitter electrode 6 made of, for example, aluminum is disposed on portions of the base region 2 and the emitter region 3 so as to provide an electrical connection between these regions. An emitter terminal E is connected to the emitter electrode 6. A base electrode 9 is located on the exposed portion of the base region 2 and is connected to a base terminal B. The base electrode 9 is surrounded by an oxide film 5. The base region 2 and the emitter region 3 are inter-digitated, and the emitter region 3 has an outer edge, i.e. interface, having a comb-teeth-shaped configuration as denoted by 7.
The emitter region 3 has a vacancy or window 4 substantially in the center of the emitter region 3 defined by an interface, i.e. inner edge, 10 with the base region 2 and through which the base region 2 is exposed at the surface of the substrate. The window 4 provides various facilities. For instance, a diode D can be formed, by using this window 4, between the base region 2 and an N.sup.+ type collector region 1 which is formed by diffusion from the other major surface, i.e., the reverse side of the semiconductor substrate S. It is also possible to form a resistor R in the base region 2 between the base electrode 9 and the emitter electrode 6, by making use of the window 4.
The known semiconductor device having the described construction is produced by forming a base region 2 at one side of the semiconductor substrate S by diffusion, followed by formation of the emitter region 3 in the base region 2 by diffusion. The base region 2 and the emitter region 3 form a transistor in cooperation with the collector region 1 formed by a diffusion into the reverse side of the semiconductor substrate S. A diode D is formed between the collector region 1 and the base region 2, and a resistor R is formed in the base region 2. The window 4 in the emitter region 3 also serves to increase the area of junction between the base region 2 and the emitter region 3. The window 4 defines an area in which the base region 2 is exposed to the surface of the device, and the portion of the base region 2 exposed through the window 4 is electrically connected to the emitter region 3 via the emitter electrode 6.
In the semiconductor device having a triple-diffused construction as described, the area of the window 4 formed in the emitter region 3 is so small that current is concentrated in the window 4 in the center of the emitter region 3 during the operation of the semiconductor device. Consequently, the effective operation region of the emitter is materially reduced, with the result that the transistor characteristic h.sub.FE is seriously affected.
The restriction in the area of the window 4 is also one of causes of reduction in the secondary yield breakdown strength, i.e., E.sub.S/B breakdown strength, of the transistor.